It is known that motor and incandescent lamp receive a surge current several- to ten-odd-fold higher than their rating when coupled with an ac source. Specifically, the surge current across the lamp filament is one of the major factors that causes filament snapping. Various circuits have been proposed in order to limit the surge current.
For example, Japan Utility Model Kokai Nos. 117,672/74 and 12,269/78 propose circuits, wherein a resistor with a negative temperature coefficient such as thermistor is connected in series with an incandescent lamp in order to increase the load resistance that is initially coupled with an ac source. This circuit is convenient, but has the disadvantages that it is difficult to control the time interval to obtain a stationary operation of the incandescent lamp; and that the resistor consumes non-negligible amount of electricity by converting it to heat radiation, a relatively high current continuously flows through the resistor when in operation.
In Japan Patent Kokai Nos. 215,697/84 and 230,298/84, the present inventor proposed methods that overcome these disadvantages of the known circuits, wherein a surge current-limiting resistor, connected in series with an incandescent lamp, is short-circuited after a lamp filament reaches its stationary operation by connecting in series a current-sensing means such as diode or current transformer with the resistor; connecting in parallel a thyristor with the resistor; and applying through a delay circuit the voltage across the current-sensing means immediately after a power source is coupled. These methods are effective in limiting surge current, but still have the defect that the surge current-limiting circuits used in the methods consume a relatively high electricity because the current-sensing means is still connected in series with the main circuit even after the lamp filament reaches its stationary operation.